In production and automation technology, serial bus systems are increasingly used in which the remotely arranged devices of machine peripherals such as I/O modules, transducers drives, valves and operator terminals communicate with automation, engineering and visual display systems via an efficient real-time communication system. In this arrangement, all users are networked via a serial bus, for example via a field bus, the data exchange via the bus being in general carried out on the basis of the master-slave principle.
The active bus users on the bus system, the control devices, as a rule, possess a bus access authorization and determine the data transfer on the bus. The active bus users are called the master units in the serial bus system. In contrast, passive bus users are, as a rule, machine peripheral devices. They do not receive a bus access authorization, i.e. they are allowed to acknowledge only received information signals or transfer information signals to a master unit upon request by the latter. Passive bus users are called slave units in the serial bus system.
To avoid complex cabling, field bus systems having a master-slave structure are generally arranged in ring topology, all bus users being connected to a ring-shaped transmission path. An information signal generated by the master unit is fed into the ring-shaped transmission path by the master unit and successively passes through the slave units serially connected to the ring-shaped transmission path and is then received again and evaluated by the master unit. Master-slave systems can also be designed as multi-master systems.
As a rule, the information signals are organized by the master unit into data packets which are composed of control data and useful data, for example using the Ethernet standard which provides for data packets having a length of up to 1500 bytes with a transmission speed which, at the same time, may be as high as 100 Mbit/sec. Each of the slave units connected to the ring-shaped transmission path exchanges the useful data intended for it with the Ethernet message when the Ethernet message fed in by the master unit passes through on the ring-shaped transmission path.
As a rule, the master-slave communication systems having a ring structure are configured in such a manner that the master unit has a transmitting unit as data injection point and a receiving unit as data extraction point. The individual slave units are then connected together on the transmission path to form a chain, wherein each user is connected to two neighbors and the first and last user in the chain is connected to the master unit. The data packets are transmitted in one direction starting from the master unit via its transmitting unit to the first connected slave unit and from there to the next one, until the last slave unit in the chain has been reached, and then back to the receiving unit of the master unit. Each slave unit has, for receiving the circulating data packets from the previous user, an interface with a receiving unit and, for forwarding to the following user, an interface with a transmitting unit, a processing unit being arranged between receiving and transmitting unit in order to process the data packets passing through the slave unit, i.e. to exchange the useful data allocated to the slave unit with the data packets.
Herein, the ring-shaped communication systems with master-slave structure are often designed in such a manner that the master unit forms a physical line with the slave units arranged at it, the transmission medium having a double-line structure and each slave unit having two ports with a combined transmitting/receiving unit, transmitting and receiving unit being short-circuited in the output port of the last slave unit in the transmission chain. The data packets injected into the first line by the master unit via its receiving unit are processed by the slave units on the forward path and are then simply forwarded only to the receiving unit of the master unit on the return path via the second line.
A central requirement for master-slave communication systems, particularly when they are used in production and process automation, is a high fault tolerance, that is to say the capability of the communication system to maintain the required function, i.e., for example, the production of a workpiece, in spite of the occurrence of faults. In this context, faults in the communication system which must be overcome without impairment of the process are, in addition to faults in the data packets, also the failure of entire transmission links, in particular, for example due to physical separation of the transmission medium.
To achieve a fault-tolerant master-slave communication system, particularly in the case of link faults, i.e. in the case of the failure of entire transmission sections, contra-sense double-ring structures are frequently used. Fault-tolerant master-slave communication systems having a double-ring structure, in which the master unit comprises two respective transmitting and receiving units comprising the corresponding transmitters or receivers, respectively, as well as associated control units in order to output data packets to the two communication paths, cause high hardware and switching complexity of the master unit and thus considerably increase costs. This also applies to the slave units which each comprise two processing units for processing the data packets passing through. Furthermore, in normal operation each slave unit must decide which of the two data packets passing through the two processing units are to be used for device control which greatly restricts the use of such communication systems at the required high data transmission rates. In addition, the known fault-tolerant communication systems with double-ring topology require that the master unit responds separately to a link fault and switches from normal mode into fault mode.